Marine seismic exploration is used to investigate and map the structures and character of subsurface geological formations underlying a body of water. Marine seismic data is typically gathered by towing seismic sources (e.g., air guns) and seismic receivers (e.g., hydrophones) through a body of water behind one or more marine vessels. As the seismic sources and receivers are towed through the water, the seismic sources generate acoustic pulses that travel through the water and into the earth, where they are reflected and/or refracted by interfaces between subsurface geological formations. The seismic receivers sense the resulting reflected and/or refracted energy, thereby acquiring seismic data that provides information about the geological formations underlying the body of water. Basically a towed seismic source emits a wavefield that propagates down through the earth and is reflected and/or refracted by interfaces between subsurface geological formations then propagates back to the surface where the receivers detect and discretely sample the wavefield.
Typically, an array of thousands of individual seismic receivers is used to gather marine seismic data. The seismic receivers are generally uniformly spaced and attached to streamer cables that are towed behind the marine vessel. It is known that the relative positions of the marine seismic receivers during seismic data acquisition can affect the quality and utility of the resulting seismic data. The current teaching is to construct the towing configuration for the streamers such that the ends of the streamers nearest the towing vessel, (commonly known as “near receivers” or “head of the streamers” or “leading end”) are all laterally spaced at equal distance and normally the receivers are equally spaced down the streamers. Typically contracts require that the streamers be maintained equally spaced to within 2% of nominal at the head of the streamers or the towing configuration must be adjusted to get “in spec”. In this configuration, uniform coverage of the surface and subsurface is achieved by at least the near receivers. It is known that the wavefield detected by the sensors is poorly sampled in the lateral direction (perpendicular to the streamers) in most configurations of this type because of the spacing size utilized to make the cost of the survey viable and to avoid tangles of the equipment behind the boat. Normally the spacing between streamers as compared to the station spacing down the streamer typically varies from between 4 and 32. This means that if our standard station spacing on the streamer is 12.5 meters and the spacing between lines is 100 meters, then the ratio between the two spacings is 8. Thus, if the spacing of the streamers at the head of the streamers is large the wavefield detected by the receivers is highly under sampled in the lateral direction. For clarity, “lateral” is meant to describe perpendicular to the length of the streamer.
However, unpredictable environmental forces such as currents, winds, and seas present in many marine environments can cause the relative positions of marine seismic receivers to vary greatly as they are towed through the water. Therefore, it is common for steering devices (commonly known as “birds”) to be attached to the streamer cables so that the relative positions (both lateral and vertical) of the seismic receivers can be controlled as they are towed through the water. The lateral control of the streamer positions down the streamer provides for the opportunity to intentionally flare the streamers and in so doing impact the sampling of the propagated wavefield.